This invention relates to incandescent lamps and more particularly to the gettering of such lamps.
The operating life of an incandescent lamp is greatly shortened by the presence of oxygen, carbon dioxide, and/or water vapor in the lamp atmosphere. Water vapor is particularly harmful because even trace amounts "catalyze" the evaporation of the tungsten filament coil by means of the well known "water cycle."
In the water cycle, the temperature at the tungsten coil is thermally sufficient to decompose water vapor into hydrogen and oxygen. The resulting oxygen reacts with the tungsten in the coil to form volatile oxides which migrate to cooler parts of the lamp and condense. These oxide deposits are reduced by the gaseous hydrogen to yield black metallic tungsten and reformed water, which causes the cycle to repeat.
The problems introduced by excess oxygen in incandescent lamps are likewise well known. For example, in the tungsten-halogen cycle, oxygen is the primary agent of attack on the tungsten filament. This attack may result in etching, and dendritic growth, and usually causes early filament failure. While an extremely small amount of oxygen is commonly accepted as a necessary constituent in the lamp, the amount which ends up in a finished tungsten-halogen capsule is generally recognized as being extremely variable and is always considered to be excessive. The presence of this "necessary constituent" has long been recognized as a major impediment to the fabrication of longer lived and more consistently performing tungsten-halogen lamps.
A commonly utilized solution to the oxygen problem in tungsten-halogen lamps is the introduction of one or more compounds into the lamp which will remove the excess oxygen and prevent its participation in the tungsten-halogen cycle. Such compounds are commonly referred to as oxygen getters.
Various oxygen getters and/or gettering systems have been used previously. For example, metallic getters such as tantalum, zirconium, niobium, copper, hafnium, titanium, aluminum, or various combinations thereof, have been employed as oxygen getters. Metallic getters may be attached to a portion of the filament mount within the lamp, e.g., in the form of a crimped piece of metal. These metal getters may also be incorporated as an alloy in the molybdenum leads which support the filament within the lamp.
U.S. Pat. No. 4,305,017 describes the use of the above-identified metals together with precious metals such as palladium, platinum and gold as oxygen getters. Metal flags, such as those described in the '017 patent, tend to be difficult and expensive to attach to the internal structure of a tungsten-halogen lamp. Also, some metallic getters that are used in incandescent lamps are not applicable for use in tungsten halogen lamps because they will react with the halogen and terminate the desired halogen cycle. Likewise, the fabrication of specialized getter alloys can also add considerably to the cost of manufacturing a tungsten-halogen lamp. In addition, in certain lamp types, it is desirable for the getter to be present across the entire range of locations within the lamp. Such positioning is impossible with metallic flag getters, and/or metal alloy gettering systems, which are generally limited to specific discrete locations.
Another commonly used oxygen getter for incandescent lamps is phosphorus. Phosphorus oxides which are formed by the gettering of oxygen are volatile, even at the cold spot temperatures found in hot operating incandescent lamps, including tungsten-halogen lamps.
Phosphorus can be deposited in a lamp, for example, on either the filament mount and/or the coil itself, e.g., by dipping a suspension of red phosphorus or P.sub.3 N.sub.5 in a suitable solvent. Alternatively, phosphorus can be deposited on the filament by evaporative coating of red phosphorus.
Phosphorus can also be introduced into incandescent lamps as phosphine gas (PH.sub.3), which is thermally decomposed into phosphorus and hydrogen by the heat of the coil at light-up.
Another oxygen getter which has been employed in incandescent lamps is the carbon getter. Carbon getters may be introduced to the lamp as part of a hydrogenated hydrocarbon gas or as carbon monoxide. However, in addition to deleteriously affecting filament life in certain lamp types, carbon has failed to perform as expected as an oxygen getter.
Yet another oxygen gettering system is described in U.S. Pat. No. 1,944,825. This patent teaches and claims the use of various gaseous fluoride compounds having water-absorbing properties. The list of fluoride compounds includes SiF.sub.4, BF.sub.3, AsF.sub.3, PF.sub.3, and salts thereof.
New gettering systems are constantly being developed. The present invention represents another such advance in this art.